When your body temperature is determined by your surroundings, and your metabolism is tightly tied to your temperature, life becomes an energy-balancing act when you have to counteract the cold. But how do cold-blooded creatures regulate their physiology to survive at low temperatures? Knowing that the tissues of cold adapted eurythermal marine species respond to the cold by increasing their mitochondrial content,Hans-Otto Pörtner and his colleagues from the Alfred-Wegener Institute for Polar and Marine Research decided to find out how cod populations from icy Arctic waters and the more temperate North Sea regulate their mitochondrial physiology to cope in cold conditions. Focusing on mitochondrial citrate synthase and cytochrome oxidase, two key metabolic mitochondrial enzymes, Nils Koschnick, Lars Eckerle, Magnus Lucassen and Pörtner monitored the proteins' transcription levels in fish from both populations and found that citrate synthase transcription and enzyme activity in white muscle are tightly correlated with the fishes' environmental temperature(p. 2462).
But first Koschnick and Lucassen had to clone and sequence both the cod citrate synthase and cytochrome oxidase genes before they could begin tracking the transcriptional responses under different environmental conditions. Having successfully sequenced regions of both genes, the team acclimated fish from the Arctic and North Sea populations to temperatures of 4 and 10°C, before collecting samples of two major metabolic organs, white muscle and liver, from each group and analysing the mRNA levels and enzyme activities.
While there were no readily noticeable trends in either enzymes' expression pattern in the livers of warm and cold acclimated Arctic and North Sea fish,the team found a strong correlation between the citrate synthase transcription levels and enzyme activities in the white muscle of both populations. Both populations of cold acclimated fish increased their citrate synthase mRNA levels massively when acclimated to 4°C, but the Arctic population seemed to have a much greater adaptive capacity to the cold, increasing their citrate synthase mRNA levels twice as much as the North Sea cod. The cod's citrate synthase levels were under tight temperature regulated transcriptional control and the Arctic fish had a greater expression capacity than their temperate cousins.
The team is now keen to find out what regulates the differences in both populations' citrate synthase responses. Pörtner explains that there are few other examples where two populations of the same species have different expression capacities for the same gene, and says that the cod's mitochondrial response could possibly explain the Arctic fish's low growth and high oxygen consumption rates relative to more temperate cod populations. Pörtner adds that the Arctic cod is not the only fish to experience ice cold waters;stenotherms trapped behind the Polar Front in the Antarctic Ocean are subject to a slightly colder, but highly stable, thermal environment. According to Pörtner, cold acclimation is energetically more costly for eurythermal fish adapted to a wide range of temperatures than for stenothermal fish,giving us another perspective on the alternative approaches adopted by cold-blooded creatures to survive at low temperatures.